First, hello everyone, this is my first post.
AMPCO, as I understand it is a bronze alloy. Treso is a manufacturer of AMPCO nipples.
I'm not positive, but I believe this is the manufacturer of the metal.
http://www.ampcometal.com/en/index_us.php?page=homeAnyway, I shoot a Ruger Old Army every now and then. That's the extent of my BP experience. From reading posts about that revolver, a lot of people seem to love the Treso AMPCO replacement nipples. Apparently, they are less likely to cause that ever-painfull "frozen nipple." They have two flat sides for easier removal than the stock nipples.
I've not yet used them, because when I bought the Old Army I also bought an extra dozen of their nipples and a couple of wrenches. After three years, I've only used one extra nipple because one that I removed became partially stripped. (You know, that probably could be taken out of context.

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Anyway, I dug a bit and here's a good thread (on another site) about this type of issue. I also highly recommend the site generally for firearm/bp info.
A little help on nipple selection
http://www.thehighroad.org/showthread.php?t=450789David
David